Touch screens have become pervasive in modern electronic devices. One reason for the growing popularity is the ability for the software to generate virtual controls, such as push buttons, menu icons, and sliders. Rather than creating costly physical buttons that may wear out, hardware manufacturers can create virtual buttons on the touch screens. Therefore, interfaces of electronic devices include many virtual rather than physical buttons. These virtual buttons are found on electronic devices as diverse as smart phones and dashboards in automobiles. A user interacts with computer-generated virtual buttons on a flat screen, on which the virtual buttons have no actual presence in the third dimension. The virtual buttons have length and width, but no actual depth.
User-interface designers often use shading to indicate the shadow of a virtual button that is tappable. The user learns to interact with the virtual buttons and panels. If the user interface is internally consistent, the location of each of the virtual push buttons is consistent and the user can tap the spot without having to look directly for its exact location. At this point, the body's proprioceptive sensation, such as the position of the hand using a mouse or the position of the finger tapping a push button on a smart phone, is so well learned that the user avoids looking at the target or consciously positioning the cursor. Once this has happened, the user is said to have developed a feel for the user interface.
Touch screens have an advantage that they can act as both a display showing the user interface and a control allowing the user interface to be manipulated. Prior to touch screens, a display was a non-interactive monitor and a control was an input device such as a keyboard. A disadvantage of the touch screens is that their surfaces are perfectly flat and they furnish no tactile feedback. The traditional physical controls, such as a piano keyboard or the different shapes of the physical knobs of a car radio, offer the sensation of a three dimensional surface through user's touch. The user's fingers sense when they are touching the three-dimensional volume control and the tuning knob, or pressing a key on the keyboard. For example, without a traditional keyboard, the user must use their eyes—they will not be able to use their sense of touch on a flat screen, and as a result, they must redirect their attention from the road or from the music sheet while they use their eyes to scan the surface of the radio or keyboard. The absence of tactile feedback makes the user spend more time looking at the controls rather than focusing on their main task, such as driving the car or reading the music sheet.